In conventional cellular communication systems, a user equipment (UE) has to communicate with another UE only through the relaying of a base station, regardless of the distance between two communicating UEs. FIG. 1 illustrates the conventional communication mode, where UE1 and UE2 communicate with each other through the UTRAN (Universal mobile telecommunications system Terrestrial Radio Access Network) consisting of base station transceiver (namely Node B) and RNC (Radio Network Controller), and this communication mode is also called UP-UTRAN-DOWN mode. However, under some circumstances when the distance between two UEs who are camping on the same cell is very close, it can be a more reasonable way for them to communicate directly, rather than being relayed/forwarded by a base station. This method is the so-called peer-to-peer communication, abbr. as P2P.
FIG. 2 shows a P2P communication mode, where the dashed line represents signaling link, the solid line represents data link, and the arrowhead represents the direction of information flow. Only signaling link exists between the UTRAN and the UEs, while only data link exists between two communicating UEs. Let's suppose only resource for maintaining basic communication is needed. If a direct link is taken as a radio resource unit (having fixed frequency, timeslot and spreading code), it can be easily inferred that P2P communication mode only needs two radio resource units to maintain basic communication. If additional signaling overheads are ignored, P2P communication can save about 50% radio resource than conventional communication mode. Meanwhile, the UTRAN still keeps control over P2P communication, especially over how to utilize radio resources, such that network operators can easily charge for the radio resource used by P2P communication.
A method and system for establishing P2P communication in wireless communication networks is described in a co-pending patent application entitled “Method and System for Establishing Peer-to-Peer Communication in Wireless Communication” filed by Koninklijke Philips Electronics N.V., on 24 Feb. 2004, Applicant's Docket No. PHCN030003WO, Application Ser. No. PCT/IB2004/050137, the disclosures of which are hereby incorporated by reference. This method and system is applicable to any TDD CDMA communication system, including TD-SCDMA system.
However, when this method and system for P2P communication is applied in a TD-SCDMA system, the direct mode of P2P communication alters the UP-UTRAN-DOWN communication mode in conventional TD-SCDMA system, i.e. a UE doesn't have dedicated traffic channel to connect with the UTRAN during P2P communication procedure. As a result, the UTRAN can only overhear information between two UEs who are communicating directly. Therefore, even if the UTRAN can overhear and estimate the synchronization shift of the two communicating UEs, it's incapable of maintaining uplink synchronization just like making that by adjusting the TA (timing advance) of uplink synchronization via a special traffic burst structure in conventional communication mode.
Accordingly, when P2P communication is applied in a TD-SCDMA system that has a rigid demand for uplink synchronization, this may potentially impair uplink synchronization and thus deteriorate the system performance.
To fully describe the negative effects upon uplink synchronization caused by the application of P2P communication in TD-SCDMA systems, a brief introduction of the uplink synchronization maintenance procedure of TD-SCDMA systems in conventional communication mode will be given below.
First, when a UE is powered on, it will first search and choose a suitable cell to camp on, and read the broadcast channel for the system information. When the UE sends a call request or responds to a paging message, it will establish uplink synchronization through random access procedure, and set the initial TA (timing advance) for the uplink traffic channel according to the synchronization adjustment information acquired from random access procedure.
Second, when the UE moves, the distance between the UTRAN and the UE will vary accordingly, which means that uplink may be out of synchronization. But due to the UE working in dedicated timeslot after being allocated dedicated resource, the UTRAN can continuously estimate the time shift information about the received signal of each subscriber by evaluating the channel impulse response of each midamble contained in the traffic burst structure used by the UEs in the uplink sub-frame, wherein the midamble for each UE is different in each uplink timeslot.
Third, if needed, the UTRAN sends an SS (Synchronization Shift) command to the UE in the traffic burst impulse in the next available downlink timeslot. On receipt of the SS command, the UE adjusts the timing of its transmission in step of ±k/8 chips each M sub-frames, or just Ignores the SS command. The default value of M (from 1 to 8) and k (from 1 to 8) is broadcast in the BCH (Broadcast Channel). The value of M and k can also be adjusted during call setup procedure, or readjusted during call procedure.
From the above steps, it can be seen that a UE can acquire uplink TA to ensure uplink synchronization from the SS message sent by the UTRAN in the downlink via the traffic channel in TD-SCDMA systems.
When the UE works in CONNECT mode and uplink synchronization is lost, the above step 2 to 3 will be iterated continuously to maintain uplink synchronization until the call ends.
During the handover procedure from a TD-SCDMA cell to another TD-SCDMA cell, the UE will transmit signals in the new cell with the TA adjusted by the relative timing difference Δt between the new cell and the old one. The relative timing difference Δt can be acquired from the system information broadcast in the BCH, or is contained in handover command.
The above uplink synchronization maintenance procedure for conventional communication mode is a closed-loop uplink synchronization control procedure based upon the DPCH (dedicated physical channel) and the PUSCH (physical uplink shared channel).